1. Field of the Invention
The present invention relates to an actuator for a flow control valve to control flow in a fluid channel, and more particularly to an actuator integrally provided with a fail-safe function to mandatorily actuate a valve so as to provide a safe condition for a fluid channel when electric power supply is accidentally cut off for some reason.
2. Description of the Related Art
A flow control valve is conventionally used for gas flow control performed with a shutoff valve provided in a fluid channel of a gas supply equipment, and also for switching control of a refrigerant channel for air-conditioning. Such a flow control valve is driven by an actuator in which that the rotary motion of a motor shaft is converted into the linear motion of a valve by means of a pinion gear or a screw structure.
Recently, a permanent magnet (PM) type stepping motor, which achieves a high positioning accuracy by means of step input, is increasingly combined with a mechanism to convert a rotary motion into a linear motion so that it can be used as an actuator.
A flow control valve is used in, for example, a gas meter, which is equipped with a safety mechanism to prevent troubles. Such a safety mechanism has what is called a fail-safe function incorporated in a gas shutoff valve system, and operates such that a flow control valve is automatically shut so as to close a gas channel when power supply is stopped due to electric power failure or due to trouble of a control circuit.
FIG. 7 is a schematic structural view of a conventional gas shutoff valve system provided with such a fail-safe function (refer to, for example, Japanese Patent Application Laid-Open No. 2003-042324).
The gas shutoff valve system shown in FIG. 7 includes a valve seat 101, a valve plug 102 adapted to block the valve seat 101, a valve shaft 103 connected to the valve plug 102, a guide 104 to cause the valve shaft 103 to move linearly, a spring 105 to press the valve plug 102 toward the valve seat 101, a stepping motor 106 as a rotary device having coils 111 and a rotor 112 with an output shaft 107, a feed screw 108 as a feeding element formed on the output shaft 107, a moving block 109 engaging threadedly with the feed screw 108 so as to thrust the valve plug 102, a rotation stopper 110 to prohibit the moving block 109 from rotating, and a lid 113. When something abnormal occurs in gas supply, pulsing currents with respective phases different from each other are applied to the coils 111 so as to rotate the rotor 112, whereby the moving block 109 moves toward the lid 113, and the valve plug 102, urged by the spring 105, is caused to move together so as to block the valve seat 101 thus stopping gas from flowing through.
FIG. 8 is a schematic structural view of another conventional gas shutoff valve system (refer to, for example, Japanese Patent Application Laid-Open No. 2003-222259).
In the gas shutoff valve system shown in FIG. 8, when there is something abnormal detected in gas supply, pulsing currents with respective phases different from each other are applied to respective wires 122 of coils 121 thereby rotating a rotor 123, and then a moving block 124, which engages threadedly with a feed screw 127 fixedly connected to the rotor 123, and which has a groove engaging with a rib 126 for prohibition of rotation, is caused to linearly move in the axial direction due to the feed screw 127 rotating together with the rotor 123. A valve plug 128 is engagingly attached to the moving block 124, and when the moving block 124 moves toward a valve seat 130, a sealing pad 129 of the valve plug 128 is brought into contact with the valve seat 130. And, when the moving block 124 further moves toward the valve seat 130, a coil spring 131 is further contracted so that a spring retaining portion 131 of the moving block 124 is brought into contact with a cylinder portion 133 of a sealing pad holding member 132, and the sealing pad 129 sags and is contracted, which causes the repulsion of the moving block 124 to exceed the thrust of the feed screw 127 consequently stopping the rotation of the rotor 123. At this time, the valve plug 128 is firmly pressed against the valve seat 130 by force of the coil spring 131, and gas flow is shut off. If electric power is turned off in this state, the rotor 123 keeps its position due to holding torque, and the valve plug 128 is kept pressed against the valve seat 130 so as to keep the valve closed.
In the gas shutoff valve system shown in FIG. 7 as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2003-042324, the spring 105 to press the valve plug 102 against the valve seat 101 is provided in a gas channel 114, which makes the system complicated, and the spring 105 is difficult to replace, which results in deteriorated workability in maintenance.
In the gas shutoff valve system shown in FIG. 8 as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2003-222259, since the coil spring 131 is attached at the valve plug 128, the valve plug 128 is prohibited from downsizing. Also, the coil spring 131 must be prepared so as to fit to the configuration of each valve plug 128, which leads to an increased number of components.